The increase in carbon emissions and the rising concentration of carbon dioxide and sulphur oxides in our atmosphere has forced the consideration of the control of the emission of these gasses from stationary sources such as fossil fuel combustors. A widely accepted “zero emission” policy for carbon dioxide and the need for greenhouse gas control technologies has emphasized the need to separate carbon dioxide from combustion gases and thereby obtain a purified stream of carbon dioxide.
While separation of carbon dioxide from flue gases is a viable option, the inherent cost is high. Accordingly, a range of approaches to separating carbon dioxide by more cost-effective processes is emerging. Numerous carbon dioxide separation processes are currently being tested for their deployment in fossil-fuel-based power plants.
The known absorption processes employ physical and chemical solvents such as selexol and rectisol while adsorption systems capture carbon dioxide on a bed of adsorbent materials such as molecular sieves or activated carbon. Carbon dioxide can also be separated from other gases by condensing it out at cryogenic temperatures. Polymers, metals such as palladium and molecular sieves are also being evaluated for membrane-based separation processes. A carbon dioxide chemical looping technique has been proposed which utilizes the carbonation of lime and the reversible calcination of limestone as a means of capturing and separating carbon dioxide. Fluidized bed combustion (FBC) of carbonaceous fuels is an attractive technology in which the removal of sulphur dioxide can be achieved by injecting a calcium-based sorbent into the combustor. Lime-based materials are the most commonly employed sorbents. However the sorbent utilization in the FBC system is rather low, typically less than 45%. The low utilization of the sorbent results in significant amounts of unreacted calcium oxide in the furnace ashes. This poses an expensive as well as a potential safety risk in deactivating the remaining calcium oxide before the ashes can be safely disposed of, for example in a landfill site.
Ash produced in an FBC furnace usually contains 20-30% unreacted calcium oxide. Reactivation of the sorbent by hydration with either water or steam can improve the sorbent utilization. During hydration of the partially-sulphated sorbent, water or steam permeates the outer calcium sulphate layer and reacts with the calcium oxide in the core of the sorbent particles to form calcium hydroxide. When the reactivated sorbent particles are re-injected into the FBC furnace, the thus formed calcium hydroxide decomposes to calcium oxide becomes available for further sulphation.
Recent investigations have indicated that fly ash has a quite different behaviour compared to bottom ash. Fly ash was not shown to be reactivated by means of any hydration treatment. Also, drastic steam hydration treatment actually reduced the sulphur dioxide carrying capacity of fly ash. These results suggested that while hydration is an effective measurement for reactivating bottom ash, its efficiency for reactivating fly ash is questionable.
Limestone is typically used as a sorbent for sulphur dioxide and/or carbon dioxide capture. However, with multiple calcination/carbonation cycles to reactivate the sorbent, due to loss of pore volume in the lime-based sorbent, the absorption efficiency of the sorbent particles rapidly decreases.
In principle, the pore volume created during calcinations should be sufficient to allow more or less complete recarbonation of the calcium oxide. In practice, however, recarbonation occurs preferentially near the particle exterior, such that the surface porosity approaches zero after multiple cycles, preventing carbon dioxide from reaching unreacted calcium oxide in the interior of the particle. To reach calcium oxide in the interior of the sorbent particles, the carbon dioxide must diffuse through the carbonated layer, the result is that the reaction between the carbon dioxide and the sorbent particles gradually slows down. Sintering in each calcination cycle is probably another factor for lowering the reactivation of calcium oxide after multiple carbonation and calcination cycles. Prior art processes have attempted to find a solutions to the problems associated with the regeneration of lime-based sorbent in multiple carbonation/calcination cycles.
Huege, in U.S. Pat. No. 5,792,440, discloses the treatment of flue gases exhausted from a lime kiln to produce a high purity calcium carbonate precipitate. A source of calcium oxide is hydrated to form calcium hydroxide which is contacted with carbon dioxide to form a high purity calcium carbonate precipitate.
Rechmeier, in U.S. Pat. No. 4,185,080, discloses the combustion of sulfur-containing fuels in the presence of calcium carbonate or calcium magnesium carbonate to form calcium sulfate or calcium magnesium sulfate. The calcium oxide or calcium magnesium oxide is removed from the combustion ashes, and is slaked with water to form the corresponding hydroxides, which are recycled to the combustion zone.
Shearer, in U.S. Pat. No. 4,312,280, discloses increasing the sulphation capacity of particulate alkaline earth metal carbonates to scrub sulfur dioxide from flue gasses produced during the fluidized bed combustion of coal. The recovered partially sulfated alkaline earth carbonates are hydrated in a fluidized bed to crack the sulfate coating to facilitate the conversion of the alkaline earth oxide to the hydroxide. Subsequent dehydration of the sulfate-hydroxide to a sulfate-oxide particle produces particles having larger pore size, increased porosity, decreased grain size and additional sulfation capacity.
Malden, in U.S. Pat. No. 4,900,533, discloses the production of alkaline earth metal oxide by calcining raw alkaline earth metal carbonate. The oxide is slaked in water to form a suspension of the corresponding alkaline earth metal hydroxide, cooling the suspension and carbonating the hydroxide in suspension in water with substantially pure carbon dioxide in the presence of a dithionite bleaching reagent to form a precipitate of an alkaline earth metal carbonate. The precipitate is separated from the aqueous medium by filtration.
Kuivalaine, in U.S. Pat. No. 6,290,921, discloses a method and apparatus for binding pollutants in flue gas comprising introducing at least one of calcium oxide, limestone and dolomite into a combusting furnace for binding pollutants in the flue gas in the furnace. Water is mixed in an amount up to 50% of the weight of the recovered ash to hydrate at least a portion of the calcium oxide in the ash to form calcium hydroxide. Rheims, in U.S. Pat. No. 6,537,425, discloses adding to a pulp suspension of a medium containing calcium oxide or calcium hydroxide during the chemical process of loading with calcium carbonate fibers contained in the pulp suspension, wherein the treated pulp suspension is charged with pure carbon dioxide, which, during the progression of the reaction, converts at least a significant portion of the calcium oxide into calcium carbonate.
Although the processes using the lime-based sorbents to trap both carbon dioxide and sulphur dioxide are moderately successful, they have several disadvantages. First, due to the low efficiency of absorption of carbon dioxide and/or sulphur dioxide, the addition of fresh sorbent is required, resulting in increased operating cost. Second, the amount of sorbent is far higher than inherent chemistry requires, so that the recovered combustor ash commonly contains significant amounts of calcium oxide. Third, due to the calcium oxide content, the recovered ash wastes cannot simply be disposed of in a landfill site without further processing to destroy the calcium oxide.
While it is known that sulphur dioxide capture by limestone may be improved significantly by treatment of the limestone with sodium chloride, it is also known that the addition of salt can impact negatively on the system, leading to system corrosion and the production of toxic by products. Moreover, the cost of the salt pretreatment adversely affects the low price of raw limestone.
In view of the foregoing, there is a demand for a means of regenerating lime-based sorbents by multiple calcination/carbonation processes. In addition, there is a demand for a method of pretreating the lime-based sorbent so as to increase its capture capacity for carbon dioxide and sulphur dioxide.